Use correct symbol in graduated student key

lib/mkmemberdirectory.php

@@ -155,7 +155,7 @@ END;
     $updated->addText("$date");
     $iconkey = $header->addTextRun();
     $iconkey->addText("Icon key: ", ['bold' => true]);
-    $iconkey->addText("◊: Graduated student  |  ►: Photo permission denied");
+    $iconkey->addText("●: Graduated student  |  ►: Photo permission denied");
     $header->addText($Strings->get("You agree to use the information in this directory for homeschool use ONLY. All other purposes, such as soliciting, is strictly prohibited.", false));
 
     $header->addTextBreak(2);